A standard three-dimensional machining tool, such as one that mills castings or bores a series of holes in a workpiece, typically has a head carried on a mounting plate. The head holds the actual tool which can be a drill bit, milling cutter, grinding wheel, or the like. The mounting plate is moved in three dimensions by a mechanism typically having at least three legs each with an inner end attached to the mounting plate. Many such machines actually use a hexapod or six-leg system where relatively angled pairs of legs extending at an acute angle to each other are used to stiffen the assembly. In such a hexapod system the outer ends of the struts lie on points of a hexagon whereas the inner ends are paired closely flanking points of a triangle or vice versa. The work head itself typically has a hydraulic motor connected via flexible hoses to a pressure source and sump.
There are two main types of such machines. The first type uses legs of variable length. In a typical system each leg is formed as a threaded spindle with a head seated rotatably in an outer socket joint by which it is joined to the mounting plate and an inner end seated in another such socket joint that is formed as a nut. Rotation of these spindles by respective servomotors changes their effective lengths between the respective swivel joints, allowing the mounting plate to be tilted at virtually any angle and to advance the tool along any path or axis. The actual operation of the servomotors is through a computer that is inputted the desired movement of the tool and that calculates the strut extensions and/or shortenings needed to produce this movement.
In a typical such machine the outer strut ends are above the inner ends and the tool carried by the work head attacks the workpiece primarily from above. The workpiece, e.g. a block to receive a pattern of drilled and threaded holes or a statue to be milled or burnished, is clamped in a predetermined position and thereafter the head is moved along its path to perform the desired machining operation, normally a material-removing one.
A large bellows-type cuff serves to protect the heavily lubricated and finely fitted strut mechanism used to displace the tool head from the chips and grit that may be produced by the machining operation. This cuff has its large rear end clamped around the upper ends of the struts and its smaller front end clamped around the lower ends of the struts at the mounting plate.
With each work cycle this cuff is stretched and deformed. Since the machine is normally used in a high-end production operation, it will therefore often follow the exact same sequence of motions hundreds of times, flexing and folding the cuff in exactly the same places every time. The result is rapid failure of this expensive and difficult-to-install part. In fact the service life of the best such cuff is much less than the mean time between succeeding lubrications or servicings for the mechanism it is protecting.
In the second main type of so-called horizontal machine the mounting plate moves predominantly horizontally at the mouth or outer end of a hexagonal-section horizontal tunnel. Each tunnel wall has a respective horizontally displaceable slide carrying an outer or rear swivel joint to which is attached the rear swivel end of a respective strut whose front end is similarly attached via a front swivel to the back of the mounting plate. The struts run at a modest angle to the horizontal, all lying on an imaginary frustocone. Respective servoactuators displace the slides and strut rear ends horizontally to move the mounting plate and the tool it carries.
With such a system the software and actuators must take into account the weight of the head and tool, which can change depending on machining operation, along with the desired movement. This considerably complicates the operation of the machine since for instance when the tool is fully extended lever action puts a greater strain on some of the servomotors than when it is retracted.
In such a machine there is typically no cover or protection for the strut mechanism. Thus it is exposed to whatever dust or grit might be raised by the machining operation it is conducting.